A
rock sample, recovered by drilling 116 meters below the active seafloor
hydrothermal vent site at 26°N on the Mid-Atlantic Ridge, shows
how the rock has been altered by reactions with seawater at temperature
of about 300°C. Pieces of highly altered rock (gray) are cemented
together with minerals such as iron sulfides (gold colored) and quartz
(white).

A
rock sample dredged from the Mid-Atlantic Ridge shows how seawater
flowing between subsurface rocks alters them and cements them
together. The rocks' outer rims (gray) have been chemically changed
by interaction with hot seawater and can be easily disctinguished
from the relatively unaltered interior (brown).

Rocks
dredged up from the bottom of the sea also contained telltale clues. Most
rocks from the mid-ocean ridge are black in color, with some white and pale
green crystals. But, in the late 1960s and early 1970s, scientists found
mid-ocean ridge seafloor rocks with unusual shades of green, orange, and
brown.

Scientists figured out why these rocks were different by analyzing the minerals
in the rocks, their crystalline structure, and their chemical composition.
They concluded that the original black rocks had been transformed by chemical
reactions that could only have taken place in the presence of hot water.

Much the way detectives might use clues to reconstruct how a crime occurred,
the scientists figured out how the rocks probably formed. Cold seawater
seeped into cracks in the seafloor and was warmed by heat from below. The
hot water reacted and exchanged chemicals with seafloor rocks. The chemical
reactions changed the minerals in the rocks and added minerals to the seawater.
Because hot fluidslike hot airis lighter and rises, the scientists
predicted that mineral-rich fluids would rise back to the surface and discharge
at the seafloor. They predicted that hydrothermal vents might exist, even
before any had been seen.